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Did you happen to get the "p-code" that was retrieved pointing to a faulty fuel cap? Was it P0455?
ok- unfortunately, there are a ton of parts/sensors monitored in the EVAP system. To "correctly" diagnose this, we'll need a p-code. With this p-code I can provide a better insight as to exactly which part/sensor has the fault, and then steps to check/correct it. On some EVAP p-codes, there are even Toyota issued Technical Service Bulletins which will related defect items that may cause the code- some even covered by warranty depending on milage. Without a p-code, we're just "guessing" and that will get costly and fustrating very quickly. You can have these p-codes read for Free at any local "big chain" parts store (AutoZone, PepBoys, etc).